1. Field of the Invention
This invention relates to a process for treating waste water resulting from a cutting process in a machine plant, etc. (i.e., water containing used cutting oil composed of swarf and cutting oil, etc.).
2. Description of the Related Art
A cutting process in a machine plant, etc. uses cutting oil, and produces a large amount of waste water (a waste liquid) containing used cutting oil, i. e. cutting oil, swarf (or chips), etc. Such waste water containing used cutting oil has usually been treated by flocculation and flotation.
Japanese Patent Laid-Open No. 22547/1997, for example, describes a process which includes adding an inorganic flocculant, such as polyaluminum chloride, aluminum sulfate, ferrous chloride, and the like to a waste liquid containing oily matters to cause it to undergo a flocculating reaction, adding a polymeric flocculant, such as poly(meth)acrylamide, poly(meth)acrylic acid, and the like, to the (waste liquid after flocculated reaction to form flocs (cotton-like flocculated masses), and separating them by flotation. Japanese Patent Laid-open No. 075703/1995 describes a process which includes adding a silica type flocculant (a solution of blast furnace slag in dilute hydrochloric acid) to waste water containing a machine oil, to thereby grow a precipitate, and removing oily matter by flotation under pressure.
Although these processes using inorganic flocculants have made it possible to reduce the amount of a sludge to be thrown away, its reduction is still unsatisfactory, and a further reduction thereof is desired. Moreover, the inorganic flocculants are too costly to be suitable for use on an industrial basis.
There is also known a process which includes adding sulfuric acid having a concentration of, say, 5% to waste water containing cutting oil, swarf, etc. to raise its specific gravity and thereby float the swarf and oil, and flocculating them by using an inorganic flocculant, etc. The use of sulfuric acid is, however, likely to damage equipment, etc., though it may be effective for floating swarf and oil, and moreover, the use of an inorganic flocculant is not satisfactory for an effective reduction of sludge.